Treeview with checkboxes in Delphi and use of csPartial

I want to achieve the following (using Delphi 12.2):

A TreeView with Checkboxes and two levels (each parent node has two or more child nodes, but no grandchild nodes).

Click a parent node: it becomes checked and all its children become checked.
Click the parent again: it becomes unchecked and all its children become unchecked.
Click a child node: the child becomes checked. If all children of a parent are now checked, the parent is checked. If some of the children are checked, the parent is partially checked.
Click a child node again: the child becomes unchecked. If all children of a parent are now unchecked, the parent is unchecked. If some of the children are checked and some are unchecked, the parent is partially checked.

For a node to be partially checked, I must select csPartial in CheckStyles. However, now each checkbox (both parent and child checkboxes) cycle through three states while being clicked, whereas they should alternate between checked and unchecked. I can programmatically change the state of a node to partially checked by setting its CheckState to ncsPartial, but this only works if CheckStyles is set to csPartial, and that results in the three-state cycle.

I realize that I will need to write some code to check or uncheck all child nodes when a parent is checked or unchecked. That is no problem. I just can't figure out how to make a parent node show as partially checked without including csPartial in CheckStyles.

Solution

I just can't figure out how to make a parent node show as partially checked without including csPartial in CheckStyles.

Unfortunately, you cannot. csPartial is required.

However, you can use a combination of the TTreeView.OnCheckStateChanging and TTreeView.OnCheckStateChanged events to achieve the effect that you want, eg:

type
  TMyForm = class(TForm)
    TreeView1: TTreeView;
    procedure TreeView1CheckStateChanging(Sender: TCustomTreeView;
      Node: TTreeNode; NewCheckState, OldCheckState: TNodeCheckState;
      var AllowChange: Boolean);
    procedure TreeView1CheckStateChanged(Sender: TCustomTreeView;
      Node: TTreeNode; CheckState: TNodeCheckState);
  private
    { Private declarations }
    ChangingNodeStates: Boolean;
  public
    { Public declarations }
  end;

...

procedure TMyForm.TreeView1CheckStateChanged(Sender: TCustomTreeView;
  Node: TTreeNode; CheckState: TNodeCheckState);
begin
  if ChangingNodeStates then Exit;
  ChangingNodeStates := True;
  try
    case CheckState of
      ncsChecked, ncsUnchecked: begin
        if Node.Level = 0 then
        begin
          for var I := 0 to Node.Count-1 do
          begin
            Node.Item[I].CheckState := CheckState;
          end;
        end else
        begin
          for var I := 0 to Node.Parent.Count-1 do
          begin
            if Node.Parent.Item[I].CheckState <> CheckState then
            begin
              Node.Parent.CheckState := ncsPartial;
              Exit;
            end;
          end;
          Node.Parent.CheckState := CheckState;
        end;
      end;
    end;
  finally
    ChangingNodeStates := False;
  end;
end;

procedure TMyForm.TreeView1CheckStateChanging(Sender: TCustomTreeView;
  Node: TTreeNode; NewCheckState, OldCheckState: TNodeCheckState;
  var AllowChange: Boolean);
begin
  if (NewCheckState = ncsPartial) and (not ChangingNodeStates) then
  begin
    AllowChange := False;
    Node.CheckState := ncsUnchecked;
  end;
end;

The idea being that if the user clicks on a checked or partial node then it will be forced to unchecked, while your code is allowed to set the states to whatever you want whenever a given node changes its state.